Alloys containing rare earth metals that may be used as magnet materials, hydrogen storage alloys, and anode materials for rechargeable batteries are conventionally known to be produced in a system wherein an alloy melt, a starting material, is cooled on a rotating roll into alloy thin ribbons or thin flakes (sometimes referred to collectively as alloy flakes hereinbelow). Such alloy flakes are pulverized and used in various applications.
The manufacturing system of this type is usually configured to enable a series of steps from feeding the alloy melt onto the rotating roll, cooling and solidifying, through separating the alloy from the rotating roll to be performed in an inert gas atmosphere, in order to prevent oxidation of alloys during the production process. The alloy flakes immediately after they are cooled on and separating from the rotating roll, have not been cooled down to the room temperature, but are still at a temperature of as high as several hundred degrees centigrade. Such hot alloy flakes are instantly oxidized upon exposure to the atmosphere, and may even ignite. Thus the hot alloy flakes are placed in an airtight container in an inert gas atmosphere, and kept for usually about 24 hours until the temperature falls to the room temperature, or rapidly cooled by, for example, gas-cooling to the room temperature (JP-3201944-B).
In the fields of industry wherein alloys containing rare earth metals are utilized, the final products have been demanded to have higher performance to keep up with the rapid development in electronics or the like. This in turn creates demand for development of alloys containing rare earth metals with higher performance. Such improvement in alloy performance is achieved in particular by controlling the crystal structure of the alloys.
The crystal structure of an alloy generally depends on the thermal history of the alloy flakes during the production process. Control of the alloy crystal structure is achieved by forced-cooling the as-cast alloy flakes rapidly to the room temperature, and subsequently heat-treating the flakes in a heat treatment furnace under particular conditions. Alternatively, particularly in the field of magnet materials, an attempt is made to control the alloy crystal structure by controlling the temperature for melting the starting material, the primary cooling rate on the rotating roll, and the secondary cooling rate after the alloy flakes are separated from the rotating roll. The control of the secondary cooling rate is made by collecting the alloy flakes separated from the rotating roll in a container made of a heat insulating material, and keeping the alloy flakes in the container for a particular period of time (see, for example, JP-8-269643-A, JP-3267133-B, JP-10-36949-A, and JP-2002-266006-A).
Controlling the thermal history of alloy flakes by means of the container made of a heat insulating material has disadvantages in that the thermal history cannot be uniform over the alloy flakes. This is because many of the alloy flakes solidified right after the casting is started are brought into direct contact with the container to conduct heat, and as the casting proceeds, the alloy flakes subsequently formed are built up in the container to conduct heat between the alloy flakes in contact with one another. When, in particular, as much as several hundred kilograms or more of alloy is cast in an industrial scale, it takes a few minutes to a several tens of minutes from the start to the end of the casting, but the alloy flakes are collected from the container all at once. This causes wide difference between the flakes at the bottom and the top of the container in their retention time in the container, and a diversity of thermal history is given to the alloy flakes of the same production lot. Thus a large percentage of the alloy flakes are not given the intended crystal structure. Further, collection of the alloy flakes in a container made of a heat insulating material may slow down the cooling rate of the alloy flakes, but cannot achieve precise control of the cooling rate, raise the temperature of the alloy flakes, or maintain the temperature of the flakes at a particular level.